FIG. 1 is a cross-sectional view showing the fundamental construction of a torque converter equipped with a lockup clutch. A torque converter 10 comprises a pump section 2, a turbine section 3, and a stator 4. FIG. 1 also illustrates a front cover 21, an outer shell 22, blades 23 and an inner core 24 of the pump section 2, an outer shell 32, blades 33 and an inner core 34 of the turbine section 3, and a turbine hub 35.
Also shown in FIG. 1 are a clutch piston 5, a damper support portion 51, a damper 52, a damper plate 53 and a damper spring 54 of the lockup clutch. The outer shell 32 of the turbine section 3, the turbine hub 35 and the damper plate 53 are joined together by a rivet 55. Designated at numeral 56 is a hole located centrally in the piston 5, and through the hole 56, the turbine hub 35 extends. Numeral 7 indicates a friction lining bonded on a clutch-engaging portion of the clutch piston 5. It is to be noted that this friction lining may be bonded on the side of the front cover 21. Numeral 6 designates a coupling welded on the front cover 21 and adapted to connect the torque converter to an engine. An alternate long-and-short dash line X-X indicates a central axis of the whole torque converter.
In recent years, there is an increasing tendency toward actuating a lockup clutch even from a low vehicle-speed range to improve the fuel economy. Reflecting this tendency, slip control is increasingly performed these days. In such slip control, the lockup clutch involves a problem in that a clutch piston and a damper or the like act as a shuddering body and a resilient body, respectively, and vibrations called a shudder occur as a result of a stick-slip of a friction lining.
A friction lining is formed from fibers, a filler, a friction modifier, and a resin dissolved in a solvent. As the solvent evaporates, the resin located inside the green friction lining progressively moves together with the solvent close to the surface. A high resin-proportion layer with the resin contained at a high concentration is, therefore, formed near the surface of the friction lining. The friction lining has a high coefficient of friction when the slip rotation speed is low, but a low coefficient of friction when the slip rotation speed becomes high. In other words, the μ-ν characteristics of the friction lining have a negative gradient, thereby causing a shudder by a stick-slip when the clutch is brought into engagement. It is to be noted that the expression “slip rotation speed” as used here in means a difference in rotation speed between a member with a friction lining bonded thereon and another member having a counterpart friction surface.
It has, therefore, been a conventional practice to remove the high resin-proportion layer from the surface of the friction lining and further to subject the friction lining to cutting on the side of its outer circumference such that the surface is rendered smooth. This cutting is usually performed in automatic transmission fluid. The automatic transmission fluid, therefore, adheres to the product, leading to a deterioration in the working environment. Moreover, the need for the cutting results in higher manufacturing cost.
It was, therefore, contemplated to divide a friction lining into a radially-outer-side friction lining and a radially-inner-side friction lining, to make the proportion of a filler higher in the radially-outer-side friction lining than in the radially-inner-side friction lining to provide the μ-ν characteristics of the radially-outer-side friction lining with a positive gradient, in other words, such that the coefficient of friction becomes higher with the slip rotation speed to inhibit a shudder which takes place by a stick-slip; and further, to make higher the proportion of fibers in the radially-inner-side friction lining to provide the radially-inner-side friction lining with flexibility and to increase the sealing property and contact area of its friction surface and hence the coefficient of static friction, thereby making the lock-up capacity greater.
As a result of research, it became clear that among fillers, it is diatomaceous earth that is effective for the inhibition of a shudder. Recently, there is a tendency toward referring a combination of a filler and a friction modifier simply as “a filler”.
It was also found that, even if the proportion of fibers is not made specifically higher in a radially-inner-side friction lining than in a radially-outer-side friction lining, the coefficient of static friction of the radially-inner-side friction lining can still be made higher than that of the radially-outer-side friction lining by adjusting the proportion of the filler including a friction modifier.
FIG. 2 is a front view of the friction lining 7 bonded on the piston 5 as viewed at a friction surface of the piston 5, while FIG. 3 is a cross-sectional view taken in the direction arrows III-III of FIG. 2. FIGS. 2 and 3 show a radially-outer-side friction lining 71, a radially-inner-side friction lining 72, and a seam 73 between the friction lining 71 and the friction lining 72.
In a lockup clutch, a radially-outer-side friction lining, the thickness of which is greater than a radially-inner-side friction lining and the μ-ν characteristics of which have a positive gradient, firstly comes into contact with an opposing surface to perform slip control so that the occurrence of a shudder is inhibited. As a consequence, the slip speed is high at the radially-outer-side friction lining, resulting in the production of large heat under friction. Problems have, therefore, arisen as to the dealing of the heat and the heat resistance of the radially-outer-side friction lining.